responsible for the genetic properties of riving things, the compo- 

 nents A through D of von Neumann's automaton. These properties 

 derive from just two classes of large, information-rich molecules: pro- 

 teins and nucleic acids. The nucleic acids are the ultimate self- 

 replicating, stable, yet mutable structures of all living matter today. 

 They form the genes, the bearers of the genetic heritage, in every 

 known species. This heritage consists of durable information, largely 

 concerned, directly or indirectly, with the production of specific 

 protein molecules. The latter form most of the cellular structures and 

 the chemically active enzymes — the versatile class of highly effi- 

 cient, interactive catalysts that control the chemical activities of 

 cells, including the eventual self-synthesis of more enzymes, other 

 proteins, nucleic acids, and other key molecules. The nucleic acids 

 and proteins thus constitute an interlocking and interdependent 

 association, the genetic system. Whatever is unique about living 

 matter is inherent in this system. There is an instructing tape, the 

 nucleic acid, which directs the assembly of the universal chemical 

 tools, the enzymes. Note, though, that all the important structures 

 are not single molecules, but complexes like lipid membranes. 



The duality of the genetic system arises from the circumstance 

 that survival in the struggle for existence depends on the ability of 

 organisms to synthesize a large variety of specific proteins; those 

 proteins are highly ordered, hence, thermally improbable structures 

 that must be built up by a long sequence of individual amino acids. 

 If every generation had to discover for itself how to assemble amino 

 acids in the right order to produce the proteins it needs, survival 

 would be impossible. This information must be transmitted from 

 parent to offspring, and a mechanism for storing and copying it is 

 required. Amino acid sequences cannot be copied by any known 

 chemical scheme from a preexisting protein, but nucleotide 

 sequences can be copied from a nucleic acid. Consequently, the 

 instructions for assembling protein molecules are encoded in nucleic 

 acids. Only the latter are copied for inheritance; only the former 

 are made anew to do the work. 



As indicated above, the lengthy information sequence contained 

 in the genes was generated by random mutations in DNA, screened 

 by natural selection. The genetic specifications are thus the evolu- 

 tionary product, a record of discovered solutions to the problems of 

 survival encountered by the species in the long course of its history. 



